The present invention relates to a process for the preparation of C.sub.5 -C.sub.8 aliphatic dibasic acids by oxidation of the corresponding saturated cycloaliphatic hydrocarbons in the presence of an organic acid solvent and a metal catalyst and subsequent recycling of the partly oxidized intermediates into the oxidation mixture for further oxidation.
Adipic acid is a major article of commerce and its preparation has, therefore, attracted much attention. Consequently, many processes for the production of adipic acid have been proposed. For example, one process involves nitric acid oxidation of cyclohexanol, cyclohexanone, or mixtures thereof, which can in turn be obtained by air oxidation of cyclohexane or hydrogenation of phenol. Several of these known processes are practiced commercially, but all suffer from high costs associated with such multi-step operations and the use of nitric acid, as well as from significant environmental pollution problems caused by the discharge of ozone-depleting nitrogen oxide by-products generated during nitric acid oxidation.
Processes that have been proposed for preparing dibasic acids without the use of nitric acid include air oxidation of saturated cyclic hydrocarbons and/or corresponding cyclic ketones and/or alcohols. For example, U.S. Pat. No. 3,390,174 and British Patent 1,304,855 disclose processes requiring mixtures of two or more of these components. However, many such air oxidation processes are multistep processes having poor selectivities and requiring difficult high-cost recovery processes.
Catalytic air oxidation processes are believed to involve free radical oxidation. Such oxidations are complex systems in which many types of reactions other than oxidation can occur. Free radicals will attack any C--H bonds, regardless of form, to an extent determined by bond strength and relative concentration of the specific C-H bond. As oxidation proceeds, various oxygenated compounds form, such as alcohols, aldehydes, ketones, and acids (including difunctional compounds having these functionalittes), as well as other low molecular weight carbon compounds. All of these compounds can further react via acid catalysis or thermal ionic mechanisms to form various condensation products, the most prevalent being esters. In general, the amount of condensation products will increase as the rate of oxidation relative to ester formation decreases. Process modifications that improve the ratio of oxidation to ester formation would be expected to yield a greater amount of easily recoverable diacid. In addition, modifications that lower the rate of esterification would be expected to improve the amount of easily recoverable diacid.
Until now, however, the seemingly attractive direct oxidation routes have not provided a viable commercial process, possibly because of the complexity of the reaction residues ("bottoms") containing inter alia many different simple esters derived from the various intermediates, oxidation products, and post-oxidation products. Such complexity is not unique to saturated cyclic alkane oxidations. Complex reactions exist even for oxidations of aromatic compounds such as xylenes. The primary distinction is that bottoms from methyl-substituted aromatic oxidation (i.e., intermediates, derivatives, and the like) can be subjected to very stringent oxidation conditions that allow further oxidation to oxidation-stable aromatic acids. For example, the aromatic acid products are extremely stable to further oxidation and can be subjected to extreme conditions under which a substantial amount of even seemingly inert acetic acid would be oxidized to CO.sub.2 and water. Consequently, these aromatic acid products can be produced substantially free of oxidation bottoms, intermediates, derivatives, and the like at very high conversions of 95% or higher.
Aliphatic diacids such as adipic acid, on the other hand, are subject to further oxidation because the C--H bonds of the methylene groups in such acids can more readily undergo free radical attack and oxidation. If subjected to forcing oxidation conditions at higher conversion, the various bottoms, intermediates, and derivatives will oxidize (as do similar aromatic compounds). However, because of the relative instability to oxidation of the aliphatic acids (such as adipic, glutaric, and succinic acids, and even acetic acid under stringent conditions), these acid products and their derivatives will progressively and increasingly degrade to CO.sub.2 and water, thereby providing lower selectivity.
Single-step direct air oxidation processes for the production of dibasic acids have been proposed. However, previously known one-step processes have been attended by poor selectivities, low production rates, multi-step operation, burdensome and costly separation steps, and low ultimate overall yields of dibasic acids from the saturated cyclic hydrocarbon. For example, U.S. Pat. No. 2,223,493 discloses a process for the direct oxidation of cyclohexane to form adipic acid at a reported production rate of 3.1 wt. % per hour in a concentration of 12.4 wt. % in the oxidation effluent with an overall selectivity of 46 to 49 mole %. This oxidation was carried out using a comparatively high concentration of cyclohexane (about 61 to 63 wt. %) in acetic acid solvent in the presence of air and various catalysts at temperatures of from 95.degree. C. to 120.degree. C. until a conversion level of about 23 to 24% was achieved.
U.S. Pat. No. 2,589,648 discloses a single-step oxidation process in which acetone is used instead of acetic acid as solvent.
U.S. Pat. No. 3,231,608 discloses another single-step direct oxidation process for the production of dibasic aliphatic acids. The reference teaches that certain critical ratios of solvent and catalyst to the saturated cyclic hydrocarbon can yield dibasic aliphatic acids under mild reaction conditions, usually at production rates of adipic acid of 3.5 to 4.0 wt. % per hour and at efficiencies generally around 73 to 76 wt. %. (It may be noted that, because adipic acid is the primary product of cyclohexane oxidation that has the highest possible molecular weight, the unit "wt. percent" will be higher than the usual selectivity-indicating unit "mole percent". In general, therefore, the reported efficiencies will be from about 2.8 to as much as 5 percentage points lower on a mole percent basis.) In particular, the reference teaches that molar ratios of solvent to saturated cyclic hydrocarbon in the range of 1.5:1 to 7:1 (or more) are suitable but that molar ratios below or above this range give unsatisfactory results.
Additional references describe attempts to improve upon the process of U.S. Pat. No. 3,231,608. A general objective of these references was attainment of higher conversions of cyclohexane, which was usually achieved by lowering the starting concentration of cyclohexane, by using protracted reaction times, or by making other such changes, with the result being very low reaction rates, reduced selectivities, and expensive recovery and downstream processing. For example, U.S. Pat. Nos. 4,032,569 and 4,263,453 require a greater relative amount of cobalt(III) catalyst (and U.S. Pat. No. 4,263,453 also requires small amounts of water) but still specify essentially the same molar ratios of solvent to cycloalkane as described in U.S. Pat. No. 3,231,608. G. N. Kulsrestha et al in J. Chem. Tech. Biotechnol., 50, 57-65 (1991), similarly discloses an oxidation process that uses a relatively large excess of acetic acid and a relatively large amount of cobalt(III) catalyst. U.S. Pat. No. 4,158,739 discloses a similar preparation of glutaric acid from cyclopentane in which the molar ratio of solvent to cyclopentane must be at least 1.5:1 and the amount of catalyst is relatively higher than for the process disclosed in U.S. Pat. No. 3,231,608. In general, the use of excess acetic acid solvent at the higher molar ratios disclosed in the prior art appears to reduce the rate of adipic acid product.
Further details on a known single-stage oxidation process for the preparation of adipic acid from cyclohexane are discussed by K. Tanaka in Chemtech, 555-559 (1974), and Hydrocarbon Processing, 53, 114-120 (1974).
The complexities involved in the recovery of adipic acid are clearly evident from the above references. Although the formation of certain oxidation intermediates that are themselves oxidizable and recyclable (such as cyclohexanol and cyclohexanone) is known, difficulties associated with the compositional complexity of the "bottoms" (or residue) from the cycloalkane oxidation, whether related to controlling the formation of the bottoms or to handling their disposition, have not been resolved by the known methods.
For example, British Patent 1,304,855 discloses the direct oxidation of cyclohexanol, cyclohexanone, and cyclohexane, which is in a sense somewhat similar to oxidizing cyclohexane in the presence of recycled cyclohexanol and cyclohexanone. However, the references disclose a selectivity to adipic acid of only 54 mole % based on all oxidized cyclic six carbon compounds.
U.S. Pat. No. 3,390,174 discloses the oxidation of saturated C.sub.5 -C.sub.8 cyclic hydrocarbons in the presence of the equilibrium concentrations of the corresponding cyclic alcohols and ketones. For example, for the oxidation of cyclohexane, the reference indicates that oxidation equilibrium levels are about 14 to 24% cyclohexanol and about 30 to 40% cyclohexanone, based on the amount of cyclohexane. However, the reported selectivities of cyclohexane to adipic acid are again only about 45 to 52 mole %.
Cyclohexanol and cyclohexanone are more readily oxidized to produce lower oxidation equilibrium levels than are cyclohexanol esters, which are among the primary constituents of the bottoms. In general, for example, an alcohol is oxidized four to five times faster on a relative molar rate basis than an ester of the same alcohol, resulting in a molar concentration of esters at oxidation equilibrium that is four to five times higher than the equilibrium level of the corresponding alcohol. Thus, when using methods that depend on the presence of cyclohexanol and cyclohexanone, the buildup of esters in the bottoms would be expected to adversely affect the efficiency of adipic acid production.
The inferior results described above can generally be attributable to the difficulty of attaining good molar selectivities by the direct oxidation of cyclohexane. Molar selectivities to adipic acid in the 80 mole % range have been achieved only when using air oxidizing oxygenated cyclic compounds such as cyclohexanone and/or cyclohexanol. For example, British Patent 1,237,479. However, these methods suffer from the disadvantages of multi-step operations, which include the generally costly production of the oxygenated cyclohexanol and/or cyclohexanone by oxidation of cyclohexane at typically very low conversions and at selectivities of about 80-92 mole %. Thus, when the complexity and multiplicity of operating steps are taken into consideration, the overall process selectivity to adipic acid is at best only about 70-79 mole %.
It has now been found possible, however, to prepare dibasic acids by the oxidation of saturated cycloalkanes by a process having very desirable commercial features. In particular, it has now been found that the ultimate obtainable selectivity and overall recovery of dibasic acids prepared by the oxidation of saturated cycloalkanes can be improved by minimizing the formation of bottoms, returning intermediates and derivatives (including the complex bottoms), optionally after partial further treatment, to the oxidation environment, and working up after oxidation. Indeed, the direct air oxidation of cyclohexane can be achieved at molar selectivities virtually identical to the best air oxidation of the facilely oxidized cyclohexanone and cyclohexanol. Moreover, the oxidation of cyclohexane, according to the process of this invention, is attended by a low production of CO and CO.sub.2, which reflects the amount of degradation of the C.sub.6 structure.